1. Field of the Invention
The present invention relates to a synchronizer ring used for a synchromesh mechanism of a gear transmission in an automobile, and a manufacturing method thereof.
2. Description of the Prior Art
Referring to FIGS. 6 and 7, a synchromesh mechanism of a speed reduction gear as shown wherein the revolution speed is changed by synchronizing the rotational speed of a clutch hub sleeve of a shaft to driving a counter gear as synchronized with the rotational speed of a mesh gear. Generally, the synchromesh mechanism has the following structure.
The synchromesh mechanism comprises the clutch hub sleeve 1, a clutch hub 2, a synchronizer ring 3, a shifting key 4 and a key spring 5. The clutch hub 2 is closely fitted on the shaft 6 by splines to rotate with the shaft 6. Furthermore, the clutch hub sleeve 1 is closely fitted on the outer periphery of the clutch hub 2 by splines.
In order to change the revolutions of the gear when a change lever (not shown) is lightly shifted from a neutral position to a direction for changing the revolutions, the clutch hub sleeve 1 is moved with the shifting key 4 by a fork (not shown). Then, the end of the shifting key 4 presses the shoulder portion of the synchronizer ring 3, to thereby press a conical surface 3a of the synchronizer ring 3 against a conical surface 7a of a piece gear 7 in mesh with the other side of the synchronizer ring 3.
In consequence, due to a light friction force between these two conical surfaces 3a and 7a, the rotation of the piece gear 7 varies in accordance with that of the synchronizer ring 3, and the piece gear 7 starts to increase or reduce the speed (starts the synchronizing operation). When a force is further exerted on the clutch hub sleeve 1, the clutch hub sleeve 1 overcomes a tensile force of the key spring 5 and moves to a further extent while depressing the shifting key 4. Thus, the synchronizer ring 3 is strongly pressed against the conical surface 7a of the piece gear 7 to increase the friction force between these two members and to synchronize rotation of the clutch hub sleeve 1 and the rotation of the piece gear 7 with each other, so that the clutch hub sleeve 1 is brought into engagement with the piece gear 7, thereby changing the revolutions.
In order to obtain an appropriate friction force between the conical surface 3a of the synchronizer ring 3 and the conical surface 7a of the piece gear 7, the conical surface 3a of the synchronizer ring 3 is screw-threaded to remove lubrication oil, and after molybdenum is coated on the screw-threaded portion by thermal spraying, a grinding process is given on the top portion of the screw thread. Thus, lubrication oil is wiped from the conical surface 7a by the screw-threaded portion, to increase the friction force and to improve the durability (see, for example, FIG. 3 of Japanese Utility Model Unexamined Publication No. 55-7414).
If the grinding process is not performed, the coated surface will be largely uneven, and projecting parts of the uneven surface will firstly abut against the conical surface of the gear and come off or be deformed, thus increasing the inner diameter of the synchronizer ring. In this case, a gap G between the end surface of the gear and the end surface of the synchronizer ring on the larger-diameter side is decreased (see FIG. 7), and the end surface 3b of the synchronizer ring on the larger-diameter side is brought into contact with the end surface 7b of the piece gear 7 before the friction force between the conical surface 3a of the synchronizer ring 3 and the conical surface 7a of the piece gear 7 reaches a sufficient value. As a result, the synchronizing operation can not be carried out, and an uncomfortable phenomena such as the so-called gear noise are induced.
Also, as disclosed in Japanese Utility Model Unexamined Publication No. 49-72842, thermal spraying is performed to form a coating layer, and thereafter, the coating layer is screw-threaded. However, this method involves a drawback in that a material for thermal spraying is expensive.